Anodic coating of magnesium



Patented June 21,1949

UNITED STATES PATENT OFFICE ANODIC COATING OF MAGNESIUM N Drawing.Application October 23, 1944, Serial No. 560,037

4 Claims.

This invention relates to an improved method of producing abrasionandcorrosion-resistant coatings upon the surface of articles of magnesiumand magnesium-base alloys.

It is known that coatings can be formed on articles of magnesium andmagnesium-base alloys by anodic treatment in aqueous alkaline solutions.However, such coatings are very soft and porous, and alford littleprotection to the base metal. By the addition of certain compounds tothe alkaline bath it is possible to improve somewhat the corrosionresistance of the coatings formed, but in no case is any significantchange effected in the resistance of the coatings to mechanicalabrasion.

It is, therefore, an object of the present invention to provide ananodic treatment for ma nesium and its alloys which produces a coatingwhich, in addition to being satisfactorily resistant to corrosion byaqueous media, is also exceptionally resistant to abrasion.

This object is attained in the invention according to a process in whichthe magnesium or magnesium-base alloy article to be protected isimmersed in a solution consisting essentially of a strong base dissolvedin a mixture of water and at least one water-soluble alcohol oralcoholether, the solution being maintained at a temperature betweenabout 50 and about 150 C., and is then subjected to anodi'c oxidationwhile so immersed. As a result of this treatment, there is formed on themetal surface a dense thin coating or film, usually gray in color, whichhas a high resistance to scratching and rubbing, and is also resistantto corrosive action. This coating provides exceptionally effectiveprotection for the base metal against factors tending to attack it underordinary usage.

In so far as is known, any strong base may be employed in the treatingsolution according to the invention, although the alkali metalhydroxides, such as sodium, potassium, and lithium hydroxides aredistinctly better than other bases. Optimum hardness and uniformity ofthe coating appear to result when lithium hydroxide is used. In general,the base should be present in a concentration from at least 30 grams perliter of solution up to its solubility limit, concentrations of 90 to120 grams per liter being especially advantageous.

The water-soluble alcohol component of the solution may be a monohydricalcohol, such as methanol or ethanol, a polyhydric alcohol such asethylene glycol, propylene glycol, glycerol, or mannitol, analcohol-ether such as 2-ethoxy ethanol or diethylene glycol, or mixturesof two or more of these substances. Those alcohols and alcohol-etherscontaining at least two hydroxyl groups are preferred because of theirlower volatility, which minimizes loss by evaporation. The alcohol oralcohol-ethers are customarily added to the treating solution in aconcentration between about 50 and about 500 milliliters per liter ofsolution, values of 100 to 200 milliliters being particularly suitable.

In practicing the invention, the magnesium or magnesium-base alloyarticle to be treated is first cleaned thoroughly, as by machining,buffing, sanding, or pickling for a few seconds in a dilute aqueousmixture of nitric and sulfuric acid. With heavily oxide-coated articles,boiling-dilute aqueous chromic acid may be necessary to effect completecleaning. Grease films, if any, on the metal may be removed by solventdegreasingor cathodic alkaline cleaning.

After cleaning, the article is immersed in an aqueous alcoholic alkalisolution according to the invention, and is subjected to electrolysiswhich produces an anodic oxidation. Sufiicient electric potential,either alternating or direct, is applied between the article and anotherelectrode, such as iron or magnesium, in contact with the treatingsolution to produce a current density at the article between 1 and 100amperes per square foot, most desirably 10 to 20 amperes per squarefoot. A low potential, usually 3 to 10 volts is preferred, since atincreased potentials, say 20 to 80 volts, the coatings formed aresomewhat softer than at the lower voltages. During the electrolysistreatment the solution is maintained at a temperature between about 50and about 150 C. and usually below the boiling point at atmosphericpressure. Temperatures between and 100 0. are preferred, with to C.being most advantageous. In general, the electrolytic treatment requiresfrom 1 to 60 minutes to produce a satisfactory coating, depending on thetemperature, current density, and whether D. C. or A. C. is used, thelatter requiring about twice the time of the former. At 70 to 80 0. and10 to 20 amperes per square foot, times of to 30 minutes give optimumresults.

The frequency of the alternations when A. C. is used does not appear tobe critical, as a wide range of frequencies may be used, e. g. to 300cycles per second, 60 cycles per second being preferred.

The following example will illustrate the invention:

Example Carefully cleaned articles of Dowmetal X-alloy (a magnesium-basealloy containing 3.0 per cent aluminum, 0.2 per cent manganese, and 3.0per cent zinc, the balance being magnesium) were immersed in a watersolution containing 100 grams of lithium hydroxide per liter and 200milliliters of ethylene glycol per liter, the solution being maintainedat a temperature of 70 C. A direct current potential of 5 to 8 volts wasthen applied to the articles using an iron cathode, causing current toflow at a density of 15 amperes per square foot. Anodization in thismanner was carried out for minutes, after which the articles wereremoved from the bath, washed, and dried. They were covered with a thindense corrosionresistant gray film which had an abrasion resistance of88 grams, as measured with a Haueisen abrasiometer (see H. G. Arlt,Proc. Am. Soc. Testing Materials, 40, 967 (1940) l using Aloxite powderand an air pressure of 12 ounces per square inch.

Anodic coatings prepared in similar manner with the exception that noethylene glycol or other alcohol was present in the treating solutionhad an abrasion resistance of only one-third to one-fourth that obtainedin the present of glycol.

While magnesium articles provided with anodic coatings according to theinvention have, in addition to high abrasion resistance, a corrosionresistance entirely adequate for most uses, the corrosion resistance maybe further improved by sealin the anodic coating by suitable chemicaltreatment. Dipping the coated article in a hot dilute non-acidicsolution of a water-soluble chromate for a few minutes is especiallyefiective. Treatment with hot water or hot alkali metal hydroxidesolutions also serves to seal the anodic coating.

The coatings of the invention may be dyed any desired color simply byimmersing the coated article in an aqueous solution of a water-solubledye of the proper color. The dye diifuses throughout the coating, and isretained permanently. However, care must be taken to immerse the articlein the dye solution immediately after the anodic coating is formed,since articles which 4 have been exposed to the air for more than a fewminutes absorb the dye only superficially.

If desired, for special uses, the anodic coatings according to theinvention may be impregnated with oils, waxes, or resins. The coatingsalso serve as effective paint bases.

This application is a continuation-in-part of my copending applicationSerial No. 469,628, filed December 11, 1940, now abandoned.

I claim:

1. A method of producing a dense, thin abrasionand corrosion-resistantcoating upon articles of magnesium and magnesium-base alloys whichcomprises immersing the article in a solution consisting of water, analkali metal hydroxide in a concentration of at least 30 grams per literof solution, and a water-soluble aliphatic hydroxyl compound selectedfrom the group con sisting of methanol, ethanol, ethylene glycol,propylene glycol, glycerol, mannitol, z-ethoxyethanol, and diethyleneglycol in a concentration between 50 and 500 milliliters per liter ofsolution, said solution being maintained at a temperature between andC., and subjecting the article While so immersed to anodic electrolysisat a current density between 1 and 100 amperes per square foot and at apotential not exceeding about 10 volts.

2. A method according to claim 1 wherein the alkali metal hydroxideemployed is lithium hydroxide.

3. A method according to claim 1 wherein the water-soluble aliphatichydroxyl compound is diethylene glycol.

4. A method according to claim 1 wherein the water-soluble aliphatichydroxyl compound is ethylene glycol.

HERBERT K. DE LONG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,923,502 Prier Aug. 22, 19332,229,252 Michel et al. Jan. 21, 1941 FOREIGN PATENTS Number CountryDate 294,237 Great Britain Sept. 12, 1929 387,437 Great Britain Feb. 9,1933 429,344 Great Britain May 24, 1935 483,798 Great Britain Apr. 26,1938 513,837 Great Britain Jan. 17, 1938 594,062 Germany Mar. 9, 1934

